Memory devices are typically provided as internal, semiconductor, integrated circuits in computers or other electronic devices. There are many different types of memory including random-access memory (RAM), read only memory (ROM), dynamic random access memory (DRAM), synchronous dynamic random access memory (SDRAM), and flash memory.
Flash memory devices have developed into a popular source of non-volatile memory for a wide range of electronic applications. Flash memory devices typically use a one-transistor memory cell that allows for high memory densities, high reliability, and low power consumption. Changes in threshold voltage of the cells, through programming of a charge storage structure (e.g., a floating gate or charge trap), or other physical phenomena (e.g., phase change or polarization), determine the data state of each cell. Common uses for flash memory include personal computers, personal digital assistants (PDAs), digital cameras, digital media players, digital recorders, games, appliances, vehicles, wireless devices, cellular telephones, and removable memory modules, and the uses for flash memory continue to expand.
Flash memory typically utilizes one of two basic architectures known as NOR flash and NAND flash. The designation is derived from the logic used to read the devices. In NOR flash architecture, a logical column of memory cells is coupled in parallel with each memory cell coupled to a data line, such as those typically referred to as bit lines. In NAND flash architecture, a column of memory cells is coupled in series with only the first memory cell of the column coupled to a bit line.
As the performance and complexity of electronic systems increase, the requirement for additional memory in a system also increases. However, in order to continue to reduce the costs of the system, the parts count must be kept to a minimum. This can be accomplished by increasing the memory density of an integrated circuit by using such technologies as multilevel cells (MLC). For example, MLC NAND flash memory is a very cost effective non-volatile memory.
Multilevel cells can take advantage of the analog nature of a traditional flash cell by assigning a data state, e.g., a bit pattern, to a specific threshold voltage (Vt) range of the cell. In the industry, these data states are often referred to as “levels.” This technology permits the storage of two or more bits of information per cell, depending on the quantity of voltage ranges assigned to the cell and the stability of the assigned voltage ranges during the lifetime operation of the memory cell.
In many flash memories, both single level cell (SLC) and MLC memories, charge storage structure coupling has increased, in part due to increasing memory densities and the like. Coupling occurs between access lines (such as those lines referred to as word lines) and between data lines (such as those lines referred to as bit lines). Coupling issues between word lines of adjacent pages of a memory, such as between even and odd memory pages, depends on the bit pattern for the data that is to be programmed in the memory. When cell threshold voltages change due to programming, the changes in threshold voltage can further increase coupling effects.
For the reasons stated above, and for other reasons which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for improved compensation in memories.